Piezoresistive sensors have achieved widespread use as pressure or force measuring devices. In general, the prior art is replete with a number of patents which describe configurations and methods of fabricating a piezoresistive device. Presently, there is an increasing demand for the use of pressure transducers in various hostile environments such as high temperature, high vibration and acceleration and conductive media. As such, these devices need to be reliable but relatively economical. In any event, a dielectrically isolated semiconductor sensor piezo resistor is an attractive configuration in regard to many such uses.
In particular, there are many instances where it is necessary simultaneously to measure both an absolute and a differential pressure. Such an instance is an aircraft air speed indicator often called a pilot-static transducer. In this case, air speed is determined by measuring the "total" pressure, which is the pressure imposed on a "pilot tube", which is the pressure against the nose of the aircraft as it moves through the air, and the "static" pressure which is the atmospheric pressure surrounding the aircraft. The air speed is proportional to the difference between the "total," and "static" pressures.
In another instance, power surges in a jet engine can be detected by measuring the compressor intake pressure and the discharge pressure. The intake pressure and the pressure difference between the intake pressure and discharge pressure indicates the presence or absence of engine surge.
Another such application is the measurement of the pressure drop across a filter to determine whether the filter is allowing adequate flow. In this case, the pressure in front of the filter and the differential pressure across the filter indicate whether the filter is clean or clogged.
In all of the aforementioned instances, the transducer can be exposed to a hostile environment having a high temperature and/or high shock or vibration, and often a hostile pressure media.
There exist methods and devices which enable such measurements to be made, for instance, commonly assigned, copending U.S. patent application Ser. No. 09/338,145, filed on Jun. 23, 1999 and entitled "COMBINED DIFFERENTIAL/ABSOLUTE PRESSURE TRANSDUCER AND METHOD FOR MAKING THE SAME", the entire disclosure of which is incorporated by reference as if being set forth herein in its entirety. Conventional transducers are typically big, bulky and expensive, often requiring additional complex electronics (using capacitative transducers requires extensive electronics for signal conditioning and amplification) and in general are not suitable for use in hostile environments.
For instance, prior art semiconductor piezoresistive transducers typically require two sensor chips each having a separate metal isolation diaphragm and an oil filtered cavity to isolate it from the pressure media. In addition, prior art semiconductor, piezoresistive devices typically contain ultra-thin gold wires ball-bonded between a sensor chip and an external contact. Such gold wires may break when the transducer is exposed to high shock and vibration. Moreover, most prior art semiconductor sensors employ p-n junctions to insure isolation of the piezoresistive network from the bulk semiconductor, which is not suitable for use in high temperatures.
It is therefore an object of the present invention to produce a relatively small and economical dielectrically isolated single chip leadless sensor which is capable of simultaneously measuring absolute and differential pressure.